Electron lenses



Feb. 23, 1960 v D, SABOR ETAL ELECTRON LENSES Filed Oct. 22. 1957 United States Patent 2,926,255 ELECTRON LENSES Dennis Gabor, London, and Peter Rawsthorne Stuart, Guildford, England, assignors to National Research Development Corporation, London, England, a British corporation' Application October 22, 1957, Serial No. 691,659 Claims priority, application Great Britain October 25, 1956 4 Claims. (Cl. 25049.5)

The present invention relates to electron lenses. In the specification of co-pending patent application Serial Number 592,142, there is described a form of electron lens which serves to turn a plane fan of incident rays back upon itself into a plane parallel to the plane of incidence, but displaced relative thereto. The lens described in that specification is shaped to provide a convex electrode conguration towards the incident rays and has the effect that the angle of divergence between the emergent rays is increased in relation to that of the incident rays by virtue of the divergent characteristic of the lens itself. Such lenses may be used for at cathode ray tubes of the kind described in Patent Number 2,795,729, although other uses are also contemplated.

Because of the relatively wide angle of divergence between the emergent rays from a lens of this kind, it will readily be understood that the outer rays of a fan of rays operated upon by the lens emerge from the lens at a distance from the centre of the lens which is signiicantly greater than the distance from the centre of the lens at which they enter and unless this fact is taken into account, the emergent fan of rays produced by a plane incident fan is liable to be distorted out of true planarity.

The present invention has for its object to provide an electron lens of the kind described in the co-pending patent application above referred to in which account is taken of the relative displacements from the centre of the lens of incident and emergent rays deflected from the central position.

According to this invention an electron lens adapted to receive an incident electron beam deectable about a given centre in an incidence plane and return it in an emergence plane parallel to said incidence plane comprises a trough-shaped repeller electrode the side walls of which embrace the two said planes, a central spine electrode and a pair of anking electrodes, said electrodes defining between them, when suitably energised, an electron lens having a U-shaped optic axis embracing said central spine electrode, said electrodes being shaped so as to present curved profiles to the incident and emergent beams, wherein the cross section of the electrode assembly increases progressively from the centre towards each end of the lens and wherein the cross sectional dimensions on the emergent side at any given point substantially reproduce the cross sectional dimensions on the incident side at the point where a ray reaching said lens from said given deflection centre and emergent at the said given point enters the repeller electrode.

The invention will be more clearly understood from the following description in which reference will be made to the accompanying drawings in which:

Fig. 1 is a cross sectional view of an electron lens according to the invention taken on the plane of central axis S-S in Figs. 2, 3 and 4;

Fig. 2 is a plan view of the repeller electrode of the lens according to Fig. 1;

Fig. 3 is an elevation view from the right or rear side; and

Fig. 4 is an elevation view from the left or front side of the lens illustrated in Fig. 1.

Patented Feb. 23, 1960 The lens illustrated comprises four electrodes, a central spine 14, a trough shaped electrode 15 called the repeller, and two llanking electrodes 13. In operation the electrodes 13 and 14 are maintained at high potential while the repeller trough 15 is at low potential. In Fig. 1 there has been included a plate 16 which is one of the detlector electrodes serving to produce deection of the beam in a plane perpendicular to the paper as seen in Fig. l, that is to say, parallel to the paper in Figs. 3 and 4. The heavy line marked E in Figs. l, 3 and 4 represents typical electron beams passing through the lens. As can best be seen from Figs. 3 and 4 the floor of the trough 15 is curved to present a convex surface towards the incident part of beam E and the lower edge of the spine electrode 14 is correspondingly curved. The effect is that a divergence is added by the lens to the divergence already imposed upon the electron beam by deiiectors 16. As will be seen from Figs. 3 and 4, the result is that in passing through the lens the beam, which enters the lens at some distance from the central axis S, S of the lens, is carried an appreciable distance farther from the central axis; in other words, the point of emergence is significantly farther from the central axis than the point of incidence.

If reference is made to Fig. 2, it will be seen that the side walls of the trough shaped repeller electrode 15 are inwardly curved. This shaping of the trough walls was introduced in the form of the lens described in the prior application above referred to, in a symmetrical way in order to overcome the distortion of the plane fan of incident rays into a non-planar fan of emergent rays. It has now been found that because of the significantly greater distance from the axis at which the beam emerges compared with its distance at incidence, the shaping of this trough electrode to take care of the above mentioned distortion needs to be made asymmetrically as between the incident and emergent apertures of the lens. The way in which this asymmetry is introduced is by making the cross sections of the incident and the emergent apertures equal at the centre of the lens and shaping the emergent aperture in such a way that the cross section at the point of emergence of a skew beam corresponds to the cross section of the incident aperture at the point where the skew beam enters.

The design of a successful lens embodying this correction was as follows (the distance A between the two branches of the optical axis being taken as unity):

Rl-Radius of repeller bottom 5.2 RVRadius of spine lower edge 5.55 R5-Radius of repeller edge on front side 9.25 RB-Radius of lower edge of positive side electrode on front side 9.6

electrode 0.37 Ga-Gap between repeller and rear positive side electrode 0.37

Although the two halves of the central cross section are geometrically symmetrical, the electric lields in these halves will be slightly different since these are influenced by the shape of lthe dissimilar neighbouring portions of the trough walls. The result of this is that the beam direction will be reversed by slightly more than 180 and the beam will emerge slightly focussed. This etect may be compensated by slightly offsetting the repeller relative to the positive electrode assembly i.e. making W3 slightly greater than W4.

We claim:

1. Electron lens adapted to receive an incident electron beam deectable about a given centre in an incidence plane and return it in an emergence plane parallel to said incidence plane comprising a trough-shaped repeller electrode `the side walls of which embrace `the two said planes, a central spine electrode and a pair of flanking electrodes, said electrodes defining between them, when suitably energised, an electron lens having a U-shaped optic axis embracing said central spine electrode, said electrodes being shaped so as to present curved profiles to the incident and emergent beams, wherein the cross section of the electrode assembly increases progressively from the centre towards each end of the lens and wherein the cross-sectional dimensions on the emergent side at any given point substantially reproduce the cross- Sectional dimensions on the incident side at the point where a ray reaching said lens from said given deflection centreand emergent at the said given point enters the repeller electrode.

2. An electrostatic deflection system for an electron beam comprising a pair of deflection electrodes adapted to deflect said beam in a plane fan about a deflection centre and an electron lens adapted to receive said plane fan in an incidence plane and return it in an emergence plane parallel to said incidence plane said electron lens comprising a trough-shaped repeller electrode the side walls of which embrace the two said planes, a central spine electrode and a pair of flanking electrodes, said electrodes defining between them, when suitably energised, an electron lens having a U-Shaped optic axis embracing said central spine electrode, said electrodes being shaped so as to present curved profiles to the incident and emergent beams, wherein lthe cross section of the electrode assembly constituting said electron lens increases progressively from the centre towards each end of the lens and wherein the cross-sectional dimensions on the emergent side at any given point substantially reproduce the cross-sectional dimensions on the incident side at the point where a ray reaching said lens from said given deflection centre and emergent at the said given point enters the repeller electrode.

3. An electron lens adapted to receive an incident electron beam deilectable about a given centre in an incidence plane and return it in an emergence plane parallel to said incidence plane comprising a repeller electrode having a bottom which is curved to present a convex surface towards the incident beam and front and rear side walls which embrace the two said planes, the edge of the front side wall adjacent said incidence plane being curved in the same direction as said bottom, the edge of the rear side wall adjacent said emergence plane being straight and both of said side walls being curved inwardly toward said planes, a central spine electrode having a lower edge which is curved in the same direction as the bottom of said repeller electrode `and spaced therefrom to provide a gap through which the beam may pass, yand a pair of front and rear flanking side electrodes positioned on opposite sides of and spaced from said spine electrode above the front and rear side walls, respectively, of said repeller electrode, the lower edges of said front and rear side electrodes being curved and straight, respectively, and spaced by gaps from the edges of the front and rear side walls of said repeller electrode, said electrodes defining between them, when suitably energised, an electron lens having a U-shaped optic axis the parallel branches of which embrace said central spine electrode, said electrodes being shaped so as to present curved profiles to the incidentand emergent beams, wherein the cross section of the electrode assem- Rl-Radius of repeller bottom 5.2 R2-Radius of spine lower edge 5.55 Rg-Radius of repeller edge on front side 9.25 R15-Radius of lower edge of positive side electrode on front side 9.6

R,-Radius of repeller edge on rear side w (straight) Rg--Radius of lower edge of positive side electrode on rear side 0 (straight) Rg-Radius of front side of repeller trough 9.25 R10-Radius of rear side of repeller trough 3.7 Z-Distance from deflection centre -to bottom of repeller 6.2 W3-i-W4-Inner width of repeller trough 1.9

H-Height of repeller in the central cross section-- 0.71 Gl-Gap between repeller and spine 0.37 Gz-Gap between repeller and front positive side electrode 0.37 Ga-Gap between repeller and rear positive side electrode 0.37

4. Electron lens adapted to receive an incident electron beam deectable about a given centre in an incidence plane and return it in an emergence plane parallel to said incidence plane comprising a repeller electrode having a `bottom which is curved to present a convex surface towards the incident `beam and front and rear side walls which embrace the two said planes, the edge of the front side wall adjacent said incidence plane being curved in the same direction as said bottom, the edge of the rear side wall adjacent said emergence plane being straight and both of said side walls being curved inwardly towards said planes, `a central spine electrode having a lower edge which is curved in the same direction as the bototm of said repeller electrode and spaced therefrom to provide a gap through which the beam may pass, and a pair of front and rear flanking side electrodes positioned on opposite sides of and spaced from said spine electrode above the front and rear side walls, respectively, of said repeller electrode, the lower edges of said front and rear side electrodes being curved and straight, respectively, and spaced by gaps from the edges of the front and rear side walls of said repeller electrode, said electrodes defining between them, when suitably energised, an electron lens having a U-shaped optic axis the parallel branches of which embrace said central spine electrode, said electrodes being shaped so as to present curved profiles to the incident and emergent beams, wherein the cross section of the electrode assembly increases progressively from the centre towards each end of the lens and wherein the cross-sectional dimensions on the emergent side at any given point substantially reproduce the cross-sectional dimensions on the incident side at the point where a ray reaching said lens from said given deflection centre and emergent at thesaid given point enters the repeller electrode.

References Cited in the tile of this patent UNITED STATES PATENTS 2,444,710 Ramberg July 6, 1948 2,536,878 Fleming lan. 2, 1951 2,759,117 Hasbrouck Aug. 14, 1956 2,777,958 Le Poole Jan. 15, 1957 2,795,729 Gabor June 11, 1957 2,795,731 Aiken June 11, 1957 2,878,417 Gabor Mar. 17, 1959 

